Linux Bridge MTU bug when the VXLAN tunneling is used
Affects | Status | Importance | Assigned to | Milestone | |
---|---|---|---|---|---|
openstack-manuals |
Fix Released
|
Medium
|
Unassigned |
Bug Description
I made some tests with the ML2 plugin and the Linux Bridge agent with VXLAN tunneling.
By default, physical interface (used for VXLAN tunneling) has an MTU of 1500 octets. And when LB agent creates a VXLAN interface, the MTU is automatically 50 octets less than the physical interface (so 1450 octets) [1]. Therefore, the bridge use to plug tap of VM, veth from network namespaces (l3 or dhcp) and VXLAN interface has an MTU of 1450 octets (Linux bridges take minimum of all the underlying ports [2]).
So the bridge could only forward packets of length smaller than 1450 octets to VXLAN interface [3].
But the veth interfaces used to link network namespaces and bridges are spawn by l3 and dhcp agents (and perhaps other agents) with an MTU of 1500 octets. So, packets which arriving from them are dropped if they need to be forwarded to the VXLAN interface.
A simple workaround is to increase by 50 at least the MTU of the physical interface to harmonize MTU between interfaces. But by default (without MTU customizing), the LB/VXLAN mode have strange behavior (cannot make curl from server behind a router or execute command with verbose output in SSH through a floating IP (SSH connection works)...)
[1] http://
[2] http://
[3] http://
Changed in neutron: | |
status: | New → Opinion |
Changed in openstack-manuals: | |
status: | Incomplete → Triaged |
importance: | Undecided → Medium |
Changed in openstack-manuals: | |
assignee: | Tom Fifield (fifieldt) → Diane Fleming (diane-fleming) |
Changed in openstack-manuals: | |
assignee: | Diane Fleming (diane-fleming) → xiaosheng meng (xiaosheng-meng) |
assignee: | xiaosheng meng (xiaosheng-meng) → nobody |
The concept of a system-wide MTU setting in a configuration file is intriguing here, per your suggestion on the mailing list. Have you evaluated this option and tested it out by any chance?